The discomfort felt by the driver and the passengers of a vehicle when rolling over one or more obstacles (such as, for example, manhole covers, asphalt joins, diverse joints, gravel, etc.) entails two distinct aspects. A first aspect is of a vibratory nature and is manifested by vibrations of the floor of the vehicle, the seats, and the steering wheel. A second aspect is of an acoustic nature and is manifested by the noise created inside the vehicle by the vibrations of the various parts of the vehicle. The level of discomfort felt by the occupants of the vehicle depends in particular on the body-shell, on the suspension system, on the rolling speed, and, of course, on the type of obstacle on the ground surface.
Several methods are known to a person skilled in the art with a view to evaluating the noise/comfort performance of a new suspension system for a given vehicle and allowing him to optimize the suspension system. The optimization includes obtaining characteristics of a suspension system that affords a substantially improved noise/comfort level.
For example, with a view to evaluating and optimizing the noise/comfort performance of a vehicle equipped with a new suspension system, a person skilled in the art can implement an experimental method consisting of carrying out measurements of noise and vibrations in the cabin of a vehicle rolling on a portion of road or track, making it possible to reveal a discomfort of a vibratory nature and/or an acoustic nature in the vehicle, with this portion of road or track including on its surface one or more irregularities. However, this method requires the availability of a vehicle for evaluating various suspension systems; furthermore, it may be driven only under favourable meteorological conditions, thereby inducing a sometimes excessively lengthy vehicle immobilization time and consequently a cost overrun when fine-tuning a suspension system, since numerous iterations are often necessary. It is furthermore lengthy and irksome for the operator and prone to large measurement scatter.
According to an alternative, document EP 0 886 130 B1 describes a method for predicting the noise level in the cabin of a vehicle equipped with tyres and rolling over uneven ground exhibiting a plurality of irregularities. According to this method, a global transfer function of a vehicle (also referred to as “vehicle transfer function”) equipped with tyres is determined by directly applying to each axle (at the wheel centre) of the vehicle, when stationary, loads directed along predetermined directions (in the form of knocks). A sound recording is made inside the vehicle for each impact and this operation is repeated successively for each of the front and rear positions and for each side of the vehicle. In another test, an identical tyre rolls on a flywheel, which is provided on its rolling surface with a plurality of irregularities simulating uneven ground. In this test, the tyre is mounted with a fixed axis, and the resulting lockup loads at the wheel centre are recorded. The measured lockup loads are employed as input for a model involving the vehicle transfer function, determined as mentioned above, so as to obtain the resulting noise level inside the vehicle. This method has limits, however. In particular, for any change of a tire and wheel assembly of the vehicle it is necessary to repeat the determination of the vehicle's global transfer function.
Moreover, because the vehicle transfer function is established on the basis of trials carried out when stationary, no account is taken of the mechanical characteristics of the tyres while rolling, which, as a general rule, are substantially different from the same characteristics when stationary. It is in particular known that the vertical rigidity under dynamic stressing of a tyre when stationary is greater than the same vertical rigidity under dynamic stressing of a rolling tyre.
According to another alternative, document EP 1 200 808 B1 describes a method for predicting the noise/comfort performance of a vehicle consisting of a body-shell equipped with a suspension system and rolling over uneven ground exhibiting a plurality of irregularities. Two procedures are undertaken in this method. First of all, acoustic and vibratory measurements are performed inside the cabin of the vehicle when the suspension system is in a rolling condition, rolling over ground exhibiting one or more irregularities or types of unevenness. A second procedure includes placing the suspension system, at the levels of the points of attachment, on a rigid rig. The suspension system is in a rolling condition in a manner analogous to the first procedure (loading, pressure, speed). The rigid rig is equipped with a system for measuring the lockup loads at the level of each point of attachment of the suspension system. In the course of this second procedure, the signals of force and of moment at each point of attachment of the suspension system are recorded. The vehicle transfer function is determined by the ratio of the acoustic and vibratory level inside the vehicle and the suspension system lockup load level. For the same body-shell equipped with a prototype suspension system, it is possible to predict the acoustic and vibratory level inside the vehicle when the prototype suspension system rolls over uneven ground exhibiting one or more irregularities. By placing the prototype suspension system on the rigid rig at the levels of these points of attachment, the prototype suspension system rolls as if it was rolling over uneven ground identical to that of the on-vehicle trial (i.e., as if using an actual vehicle). The lockup loads at the levels of the points of attachment are measured. By multiplying the vehicle transfer function by these lockup loads of the prototype suspension system, the acoustic and vibratory level of the body-shell furnished with the prototype suspension system rolling over uneven ground is evaluated. This method also has limits that can, for suspension systems of different structure, for example, at the level of a structure of a tyre of identical dimension, provide different noise levels from those obtained on the basis of trials carried out with an actual vehicle equipped with these various suspension systems and rolling over uneven ground.
In particular, the vehicle transfer function is determined for a reference suspension system. This reference suspension system has its own mechanical behaviour, which influences the vehicle transfer function. A prototype suspension system has its own mechanical behaviour, which may influence the vehicle transfer function in a different manner.
Finally, document WO 2005/071385 A1 describes a method for determining operational loads at the wheel centre between the suspension system, reduced to its simple tire and wheel assembly, and the body-shell. These operational loads determine a new vehicle transfer function, making it possible to predict the noise/comfort performance of a vehicle furnished with this tire and wheel assembly rolling over uneven ground exhibiting a plurality of irregularities.
In a first step, a measurement of lockup loads of the tire and wheel assembly is carried out on a rollway furnished with uneven ground exhibiting a plurality of irregularities. This rollway is equipped with a dynamometric hub for measuring the lockup loads at the wheel centre in three perpendicular directions corresponding to the vehicle's reference frame. When the tire and wheel assembly rolls over uneven ground, the lockup loads resulting from this rolling are recorded with the aid of the dynamometric hub.
A functional model of the suspension system is thereafter determined, in a second step, characterized by non-suspended masses as well as stiffnesses and dampers in the three directions of the vehicle reference frame.
Conventionally, this functional model is defined in the directions of the vehicle's reference frame and the couplings between the perpendicular directions are neglected. Moreover, only forces in the three perpendicular directions are taken into account. The identification of the parameters of this functional model is performed by measurements on the vehicle. By linking this functional model of the suspension system to a model of the tyre, a switching matrix (Hp) for switching between lockup loads and operational loads at the wheel centre is determined. The estimation of the operational loads at the wheel centre of the tire and wheel assembly is obtained by multiplying the switching matrix (Hp) by the lockup loads in the first step.
In a third step, acoustic and vibratory measurements are performed inside the cabin of the vehicle when the tire and wheel assembly is rolling over ground exhibiting a plurality of obstacles.
In a fourth step, a new vehicle transfer function is determined by using a ratio of the acoustic and vibratory level inside the vehicle and a level of operational loads of the tire and wheel assembly.
For the same body-shell equipped with a prototype tire and wheel assembly, it is possible to predict the acoustic and vibratory level inside the vehicle when the vehicle furnished with this prototype tire and wheel assembly rolls over uneven ground having several irregularities. By placing the prototype tire and wheel assembly on the rigid rig at a level of the wheel centre, the tire and wheel assembly rolls as if rolling over uneven ground identical to that of the on-vehicle trial. Lockup loads at the level of the wheel centre are measured. By multiplying the switching matrix (Hp) by the lockup loads, operational loads of the tire and wheel assembly at the wheel centre are evaluated. By multiplying the vehicle transfer function by these operational loads of the prototype tire and wheel assembly, the acoustic and vibratory level of the vehicle rolling over uneven ground is evaluated.